1. Field of the Invention
This invention relates generally to two-cycle internal combustion engines, and more particularly to two-cycle internal combustion engines in which the air-fuel mixture may be provided both to the crankcase for displacement to the combustion chamber by the downward moving piston through a transfer port, which further includes a boost port having associated therewith a reed valve to permit air-fuel mixture to flow directly from the inlet channel to the cylinder above the piston crown.
2. Description of the Prior Art
Two-cycle internal combustion engines are well known and have been highly developed. In one of the more basic embodiments, a two-cycle engine involves an inlet channel communicating with the crankcase of the relatively standard engine having a piston movably mounted in a cylinder and reciprocated by a rotating crankshaft. A vacuum is produced in the crankcase (as the piston moves in an upstroke direction) thereby drawing an intake mixture into the crankcase. As the piston moves in a downstroke direction, the pressure in the crankcase increases and the air-fuel mixture is displaced into the cylinder above the piston crown through a transfer port extending between the crankcase and the cylinder above the downward displaced piston. In order to preclude reverse flow of the air-fuel mixture out of the inlet channel, timing means, which may be the piston skirt closing a port, a reed valve, or rotary valve are provided. Otherwise, as is well known, the two-cycle engine provides a power stroke upon each downstroke with the exhaust gases first being expelled under the relatively high pressure of the combustion products, and a fresh air-fuel charge being immediately drawn therein by, for instance, displacement from the crankcase through the above-mentioned transfer port.
It is also well known that it is difficult to provide for optimum efficiency and power of a two-cycle engine under varying operating conditions. For instance, the specific timing of a piston port controlled crankcase timing means is necessarily fixed. Accordingly, if a piston seals off the intake channel early, the intake charge into the crankcase is severely limited at high operating rates. On the other hand, given the fixed timing of such an arrangement, if the piston seals the intake channel late, air-fuel mixture will be expelled back into the intake channel at low engine operating speeds. This phenomenon is a result of the differing characteristics of the intake air-fuel charge as the operating speed of the engine increases. At low operating speeds, such as upon starting, flow of the intake air-fuel mixture closely conforms to the timing means openings and pressure differentials. On the other hand, during operation at high engine speeds, substantial inertia is displayed by the air-fuel mixture. Thus the mixture does not immediately flow to obviate a pressure differential between the intake channel and the crankcase, and, similarly, maintains flow substantially beyond the existance of such a pressure differential as a result of the inertia of the flowing stream of air-fuel mixture.
To provide for the varying requirements of crankcase inlet timing under varying operating conditions, a reed valve has been employed in many instances. This arrangement is essentially a one way check valve which permits air-fuel intake mixture to flow upon demand in one direction and be precluded from reversing the direction of flow. When a vacuum condition exists in the crankcase, the reed valve will open and permit flow into the crankcase, but will close promptly upon the reversal of the pressure differentials which promote flow from the crankcase through the intake channel. Thus it will be recognized that the reed valve provides for a somewhat variable duration of the timing means controlling flow into the engine crankcase. Unfortunately, the reed valve is restrictive of air-fuel mixture flow under high operating speeds. When this is compounded by the inherent flow restriction of transfer ports, etc., the end result is that the reed valve provides for enhanced torque and power at low and mid-range operating speeds, but tends to compromise high speed engine efficiency.
One improvement upon the simple two-cycle engine described above, is that in which a reed valve is employed to control fuel flow into the crankcase, but a second intake channel is provided which communicates directly with the crankcase with piston skirt timing. Thus, the second channel is controlled and timed in a fixed manner by the piston skirt, while the primary channel is controlled by a reed valve. Accordingly, unobstructed flow past the piston skirt is provided as well as extended flow through the reed valve at mid operating speeds. Again, high speed operating is compromised by the inflexible timing of the piston skirt control and the inherent restrictions of the reed valve and transfer ports. However, enhanced mid-range operation is realized.
An even more useful arrangement is disclosed in U.S. Pat. No. 3,687,118. In this arrangement, a reed valve is provided between the air-fuel mixture providing means, i.e., a carburetor, and the crankcase. However, downstream of the reed valve, an auxiliary scavenge port is provided to permit flow directly into the cylinder above the piston crown and a piston controlled intake port is provided into the crankcase. In effect, this arrangement defines the entire volume downstream of the reed valve as an extension of the crankcase in the event reverse flow is induced prior to closing of the crankcase port. Thus some portion of the reverse flow may be directed to the auxiliary scavenge port and into the combustion chamber. Since the intake channel is piston skirt timed, the intake channel will be isolated from the crankcase prematurely at higher operating speeds. The air mixture continuing to flow at higher operating speeds as a result of inertia may be directly transferred through the auxiliary scavenging port to the cylinder above the piston crown. While this provides a useful improvement, again the intake channel is encumbered with reed valves restricting flow and the intake channel is also subject to the fixed timing of piston skirt control at the crankcase.
A variant but interesting embodiment of the concept of U.S. Pat. No. 3,687,118 is shown in FIG. 6. In this approach, only air is transferred from the crankcase to the combustion chamber and a fuel injection mechanism is provided in the auxiliary scavenging port. This requirement necessitates the inclusion of a reed valve to isolate the inlet channel from the auxiliary scavenging port. However, the complexity of a fuel injection system tends to defeat the efficiency of the two-cycle engine and still the air flow is subject to the restriction of reed valves located upstream of the auxiliary scavenging port. In fact the auxiliary scavenging port is subjected to the restriction of two reed valves in series.
U.S. Pat. No. 3,916,851 discloses an engine utilizing parallel independent intake means to provide stratified charge operation of a two-cycle engine.
In summary, while two-cycle engines are simple and can be collectively tuned to provide for easy starting, excellent fuel economy, extremely high power outputs etc., one of such desirable features is usually gained only through severly compromising another. For instance, high rpm, high power outputs, usually require piston skirt timing (or other fixed timing means such as rotary valves) of rather extreme duration. As a result, starting is often difficult and at mid-range the engine may "four cycle" in an inefficient manner. Only in the relatively narrow tuned high rpm area is the engine efficient.